1. Field of the Invention
The present invention relates to a particle beam irradiation system and a method of adjusting an irradiation field forming apparatus. More particularly, the present invention relates to a particle beam irradiation system suitable for use as a particle therapy system for irradiating a charged particle beam, such as a proton or carbon ion beam, to a diseased part (cancer) for treatment, a material irradiation system for irradiating a charged particle beam to materials, a food irradiation system for irradiating a charged particle beam to foods, and a radio isotope producing system utilizing a charged particle beam.
2. Description of the Related Art
A known particle therapy system comprises a charged particle beam generator, a beam transport system, and a rotating irradiation apparatus. The charged particle beam generator includes a synchrotron (or a cyclotron) as an accelerator. A charged particle beam (hereinafter referred to as an “ion beam”) accelerated by the synchrotron to a level of setting energy reaches the irradiation apparatus through the beam transport system (hereinafter referred to as a “first beam transport system”). The rotating irradiation apparatus comprises an irradiation-apparatus beam transport system (hereinafter referred to as a “second beam transport system”), an irradiation field forming apparatus, and a rotating apparatus (gantry) for rotating the second beam transport system and the irradiation field forming apparatus in union. After passing the second beam transport system, the ion beam is irradiated to a diseased part (cancer) in the body of a patient through the irradiation field forming apparatus.
The irradiation field forming apparatus serves not only to shape the ion beam extracted from the charged particle beam generator in match with the three-dimensional shape of the diseased part as an irradiation target, thereby forming an irradiation field, but also to adjust the irradiation dose in the irradiation field. As a method for irradiating the desired irradiation dose in match with the shape of the irradiation target, there is known a double scattering method in which two types of scatterers arranged in a spaced relation with a certain distance between them in the axial direction of the ion beam are employed to make the irradiation dose of the ion beam uniform by utilizing the fact that the ion beam after passing through the scatterer has a substantially normal distribution (see, e.g., FIG. 36 in p. 2081 of Non-Patent Reference 1: REVIEW OF SCIENTIFIC INSTRUMENTS, Volume 64, Number 8 (August 1993), pp. 2074–2086). As other irradiation methods, there are also known a wobbling method (see, e.g., Patent Reference 1: JP,A 10-211292 and Patent Reference 2: JP,A 2000-202047) and a scanning method (see, e.g., Patent Reference 3: JP,A 10-199700), in which two scanning magnets are used to make the irradiation dose of the ion beam uniform.
In those irradiation methods, a Bragg peak spreading-out device (SOBP device) is employed to make the irradiation dose of the ion beam uniform in the direction of depth of the diseased part in the patient body (i.e., in the irradiating direction of the ion beam). The SOBP device is of a structure having areas that differ in thickness and are formed on a plane perpendicular to the direction of advance of the ion beam. By causing the ion beam to pass through the areas having different thicknesses, the ion beam has plural energy components. Also, by setting a size of the area having each thickness, a weight of each energy component of the ion beam is determined. Then, a dose distribution having high uniformity in the direction of advance of the ion beam is formed by superimposing those plural energy components so as to properly adjust the dose distribution. Known examples of such an SOBP device include a ridge filter (see FIG. 31 in p. 2078 and FIG. 41 in p. 2084 of Non-Patent Reference 1), a range modulation wheel (see FIG. 30 in p. 2077 of Non-Patent Reference 1), etc.
Because those SOBP devices have, as described above, the structure having areas that differ in thickness and are formed on a plane perpendicular to the direction of advance of the ion beam, the scattering intensity of the ion beam in the SOBP device differs depending on the areas through which the ion beam passes. To suppress the influence of the scattering, it has hitherto been proposed to arrange the SOBP device at a position within the irradiation field forming apparatus on the most downstream side in the direction of advance of the ion beam. Alternatively, when the SOBP device is arranged on the upstream side, a scattering compensator is disposed to compensate for differences in the scattering intensity of the ion beam among the respective areas of the SOBP device.